The present invention relates to an image forming apparatus and an image forming method using a transparent laminated film on which a color toner image is formed by an electrophotographic printing method or an electrostatic printing method. More particularly, the present invention relates to an image forming apparatus and an image forming method for forming a color image on a transparent laminated film which is to be placed on an overhead projector (OHP).
FIG. 5 is a cross-section of an electrophotographic printing apparatus, capable of forming a full-color image. In FIG. 5, the electrophotographic printing apparatus comprises a transfer material conveyance system located from one side to the substantial center of the apparatus main body 100, a latent image forming unit provided adjacent to a transfer drum 8 which is a part of the transfer material conveyance system, and a rotation-type developing unit serving as developing means, located adjacent to the latent image forming unit. The aforementioned transfer material conveyance system comprises: transfer material feeding trays 101 and 102 which are detachably inserted to openings provided on one side (right side in FIG. 5) of the apparatus main body 100; transfer material feeding rollers 103 and 104 provided substantially directly above the trays 101 and 102; feeding guides 4A and 104b comprising feeding rollers 106, which are located adjacent to the feeding rollers 103 and 104; a transfer drum 8 rotatable in the direction of an arrow in FIG. 5; a conveyance belt 15 provided adjacent to a separation claw 14; and a fixer 16 which is provided near the end of the conveyance belt 15 in the conveyance direction, and is provided adjacent to a discharge tray 17 which extends from the apparatus main body 100 and is detachable from the apparatus main body 100. The transfer drum 8 is located adjacent to the feeding guide 104b. Near the periphery of the transfer drum 8, an abutting roller 7, a gripper 6, a transfer-material separation charger 12, and a separation claw 14 are provided in the upstream-to-downstream direction of the rotation of the transfer drum 8. In the interior periphery of the transfer drum 8, a transfer charger 9 and a transfer-material separation charger 13 are provided.
The latent image forming unit comprises: the photosensitive drum 2 serving as an image transfer body, whose outer periphery abuts against the outer periphery of the aforementioned transfer drum 8, and is rotatable in the direction of an arrow in FIG. 5; image exposure means, such as a laser-beam scanner, for forming an electrostatic latent image on the periphery of the photosensitive drum 2; and image exposure reflection means such as a polygon mirror. Near the periphery of the photosensitive drum 2, an electrostatic charger for discharge (hereinafter referred to as discharger) 10, cleaning means 11, and a primary electrostatic charger 3 are provided in the upstream-to-downstream direction of the rotation of the photosensitive drum 2.
The rotation-type developing unit comprises: a rotational body 4a; and an yellow developer 4Y, a magenta developer 4M, a cyan developer 4C and a black developer 4BK which are incorporated in the rotational body 4a and enable to visualize the electrostatic latent image formed on the periphery of the photosensitive drum 2 at a position facing the periphery of the photosensitive drum 2.
An image forming sequence of the image forming apparatus having the above-described construction is briefly described, taking a full-color mode as an example. When the photosensitive drum 2 is rotated in the direction of the arrow in FIG. 5, a photosensitive material on the photosensitive drum 2 is uniformly charged by the primary electrostatic charger 3, then image exposure is performed by a laser beam E which is modulated by yellow image signals of an original image (not shown). By this, an electrostatic latent image is formed on the photosensitive drum 2, and the electrostatic latent image is developed by the yellow developer 4Y which is positioned at a predetermined developing positions by rotation of the rotational body 4a. 
Meanwhile, a transfer material conveyed through the feeding guide 4A, feeding rollers 106, and feeding guide 104b, is gripped by the gripper 6 in a predetermined timing, and is electrostatically wrapped around the transfer drum 8 by the abutting roller 7 and electrodes facing the abutting roller 7. While the transfer drum 8 rotates in the direction of an arrow in FIG. 5 in synchronization with the photosensitive drum 2, the image developed by the yellow developer 4Y is transferred by the transfer charger 9 at the position where the periphery of the photosensitive drum 2 abuts against the periphery of the transfer drum 8. The transfer drum 8 continues its rotation, preparing to transfer the next color (magenta in FIG. 5).
The photosensitive drum 2 is discharged by the discharger 10 and cleaned by the cleaning means 11. Then, the photosensitive drum 2 is charged again by the primary electrostatic charger 3 and the above-described image exposure is performed according to the subsequent magenta image signals. The above-described image exposure is performed on the photosensitive drum 2 according to magenta image signals. The rotation-type developing unit rotates while an electrostatic latent image is formed on the photosensitive drum 2 according to the magenta image signals by the above image exposure. The magenta developer 4M is positioned at a predetermined developing position and a predetermined magenta development is performed. Next, the above-described process is performed with respect to cyan and black and completes the transfer process for four colors. The four-color toner image formed on the transfer material is discharged by the discharger 10 and charger 13. The gripper 6 releases the transfer material, the transfer material is then separated from the transfer drum 8 by the separation claw 14 and is transferred to the fixer 16 by the conveyance belt 15 to be fixed by heat and pressure. As a result, a series of full-color print sequences is completed and the necessary full-color print image is formed. The fixer 16 includes a surface-lubricant coating mechanism which promotes separation of an OHP film from a fixation member. However, it is a recent trend to use less amount of the surface lubricant.
Next, toner used in the electrophotographic apparatus is described.
Toner for a color electrophotographic printing apparatus requires excellent melting and color mixture characteristics when heat is applied. Thus, toner having a sharp-melt characteristic is preferable where the softening point is low and melting time is short.
The use of a sharp-melt toner improves the color reproducible range of an original image, and enables to achieve a colored copy consistent with a multiple-color original image.
The foregoing sharp-melt toner is manufactured by melting and mixing, e.g., polyester resin, styrene-acrylic resin, colorant (dye, sublimation-type dye), charge control agent and the like, then grinding and classifying them. If necessary, a process of adding various external agents to the toner may be added.
For color toner, it is particularly preferable to employ toner utilizing polyester as a binding resin, taking into account fixation and sharp-melt characteristics. The sharp-melt polyester resin is a high molecular compound including ester bonding in the principal chain of a molecule in which diol compound and dicarboxylic acid are synthesized.
Particularly, because of its sharp melting characteristic, it is preferable to use polyester resin, represented by the following formula (1) (R is ethylene or propylene radical; x and y are respectively a positive integer which is 1 or more; and the average value of x+y is 2 to 10), where a bisphenol derivative or its substituent as a diol component, and a carboxylic acid compound (e.g., fumaric acid, maleic acid, maleic anhydride, phthalic acid, terephthalic acid, trimellitic acid, pyromellitic acid or the like) comprising carboxylic acid of bi- (or more) valence or its acid anhydride or its sub-alkyl ester, are at least copolymerized. 
The softening point of the sharp-melt polyester resin is 60xc2x0 C. to 150xc2x0 C., preferably 80xc2x0 C. to 120xc2x0 C. The softening characteristic of toner, having the above-described sharp-melt polyester resin as binding resin, is shown in FIG. 6.
Herein, a Flow-Tester CFT 500 (manufactured by Shimadzu Corporation) is used for the purpose of an experiment. Assume that a die (nozzle) has a diameter of 0.5 mm and a thickness of 1.0 mm, and an extrusion load of 50 kg is added to the toner. At the initial setting temperature of 80xc2x0 C., the toner is preheated for 300 seconds. Then, the temperature is uniformly raised at the rate of 5xc2x0 C./minute. The curve representing the amount of plunger fall and the temperature is obtained (hereinafter referred to as a softening curve S). The sample toner used is fine powder precisely measured to 1 g-3 g. The cross section of the plunger is 10 cm2.
FIG. 6 shows the curve obtained as the softening curve S. Along with the uniform temperature rise, toner is gradually heated and eventually begins to flow out (plunger descent Axe2x86x92B). As the temperature increases further, the melted toner flows out in a greater amount (Bxe2x86x92Cxe2x86x92D). Finally, the plunger stops falling and the toner flow stops (Dxe2x86x92E).
The height H of the softening curve S indicates the amount of the entire flow. The temperature T0 for H/2, corresponding to the point C, indicates the softening point of the toner.
The above measurement method can be similarly applied to measuring a heat melting characteristic of resin for forming binding resin or a second transparent resin layer.
The sharp-melt toner or resin satisfies the following condition:
T1=90xc2x0 C. to 150xc2x0 C., |xcex94T|=|T1-T2|=5xc2x0 C. to 30xc2x0 C.
where T1 indicates the temperature when the melt viscosity is 105 cp, and T2 indicates the temperature when the viscosity is 5xc3x97104 cp.
The sharp-melt toner or resin, having the above-described temperature versus melting viscosity characteristics, is characterized by extremely sharp viscosity decrease caused by heating. Such decrease in viscosity enables appropriate mixing between the top toner layer and the bottom toner layer, and enables rapid enhancement in transparency in the toner layers, resulting in excellent subtractive mixture.
Lately, less use of the surface lubricant is the trend for improving the quality of a projected image of an OHP film.
However, in the above-described conventional example, since the projection image quality has been improved to brighter and more vivid colors, the deterioration in image quality caused by optical interference between the optical portion of the projector and an OHP image become more conspicuous.
The deterioration in image quality is now described with reference to FIG. 4. FIG. 4 is an explanatory view showing a mechanism of occurrence of interference stripes. In a case where toner images formed as a binary image are periodically arrayed on the OHP film, the OHP film acts as a diffraction grating. In addition, a Fresnel lens of the projector also acts as a diffraction grating. Therefore, because of the film and the lens arrangement, scattered light from the two gratings interfere with each other, generating difference in light intensity. The difference in intensity of light causes light-and-dark stripes in a projected image. The object of the present invention is to achieve an image forming apparatus which eliminates such light-and-dark stripes in a projected image of an OHP film. The present invention has a further object to provide an image forming apparatus and method having the new functions.
In order to achieve the above objects, the image forming apparatus and image forming method according to the present invention achieves a projected image of better quality where a formed image does not act as a diffraction grating and where optical interference does not occur between an optical system of a projector and the formed image in a case where a light-transmitting transparent film is selected as a printing material.
More specifically, an image forming apparatus according to the present invention is characterized by comprising: conveyance means for conveying a transfer material; transfer material identifying means for identifying a type of transfer material conveyed by the conveyance means; scan-line density changing means for changing scan-line density of a laser beam based on an identified result; pulse number modulation control means for modulating an output pulse number of the laser beam based on the changed scan-line density; latent image forming means for forming a latent image at the changed scan-line density, in accordance with the type of transfer material; and developing means for developing the formed latent image.
Furthermore, an image forming apparatus according to the present invention is characterized by comprising: conveyance means for conveying a transfer material; transfer material identifying means for identifying a type of transfer material conveyed by the conveyance means; amplitude modulation control means for modulating an output pulse amplitude of a laser beam for latent image formation, in accordance with an identified result; latent image forming means for forming a latent image charged at multivalue levels based on irradiation of the laser beam where amplitude is modulated; and developing means for developing the formed latent image.
Furthermore, an image forming apparatus according to the present invention is characterized by comprising: conveyance means for conveying a transfer material; transfer material identifying means for identifying a type of transfer material conveyed by the conveyance means; threshold value calculating means for calculating a threshold value for density data conversion in accordance with an identified result; density level converting means for forming a converted image where density of an original image has been converted by comparing the density of the original image with the calculated threshold value; pulsewidth modulation control means for modulating an output pulsewidth of a laser beam in accordance with the converted density data; latent image forming means for forming a latent image based on irradiation of the laser beam where pulsewidth is modulated; and developing means for developing the formed latent image.
Moreover, an image forming method according to the present invention is characterized by comprising the steps of: a conveyance step of conveying a transfer material; a transfer material identifying step of identifying a type of transfer material conveyed in the conveyance step; a scan-line density changing step of changing scan-line density of a laser beam based on an identified result; a pulse number modulation control step of modulating an output pulse number of the laser beam based on the changed scan-line density; a latent image forming step of forming a latent image at the changed scan-line density, in accordance with the type of transfer material; and a developing step of developing the formed latent image.
Furthermore, an image forming method according to the present invention is characterized by comprising the steps of: a conveyance step of conveying a transfer material; a transfer material identifying step of identifying a type of transfer material conveyed in the conveyance step; an amplitude modulation control step of modulating an output pulse amplitude of a laser beam for latent image formation, in accordance with an identified result; a latent image forming step of forming a latent image charged at multivalue levels based on irradiation of the laser beam where amplitude is modulated; and a developing step of developing the formed latent image.
Furthermore, an image forming method according to the present invention is characterized by comprising the steps of: a conveyance step of conveying a transfer material; a transfer material identifying step of identifying a type of transfer material conveyed in the conveyance step; a threshold value calculating step of calculating a threshold value for density data conversion in accordance with an identified result; a density level converting step of forming a converted image where density of an original image has been converted by comparing the density of the original image with the calculated threshold value; a pulsewidth modulation control step of modulating an output pulsewidth of a laser beam in accordance with the converted density data; a latent image forming step of forming a latent image based on irradiation of the laser beam where pulsewidth is modulated; and a developing step of developing the formed latent image.
Moreover, an image forming apparatus according to the present invention is characterized by comprising: identifying means for identifying a type of print medium; and control means for controlling printing operation for forming an image on the print medium in accordance with an identified result of the identifying means.
Furthermore, an image forming method according to the present invention is characterized by comprising the steps of: an identifying step of identifying a type of print medium; and a control step of controlling printing operation for forming an image on the print medium in accordance with an identified result in the identifying step.
According to an aspect of the image forming apparatus of the present invention, the scan-line density is 250 Lines/inch or more in a case of using a transparent film as the transfer material.
According to an aspect of the image forming method of the present invention, the scan-line density is 250 Lines/inch or more in a case of using a transparent film as the transfer material.
According to an aspect of the image forming apparatus of the present invention, the identifying means determines whether the print medium is an OHP film or a regular sheet of paper.
According to an aspect of the image forming apparatus of the present invention, the controlling of printing is to control scan-line density at the time of image formation.
According to an aspect of the image forming apparatus of the present invention, the control means controls an output pulse of a laser beam at the time of image formation.
According to an aspect of the image forming apparatus of the present invention, in a case where the identifying means determines that the print medium is an OHP film, the control means performs image formation at print density such that the OHP film does not act as a diffraction grating due to a formed image.
According to an aspect of the image forming apparatus of the present invention, the print density where the OHP film does not act as a diffraction grating is 250 Lines/inch or more.
According to an aspect of the image forming apparatus of the present invention, the control means controls a thickness of development toner at the time of image formation.
According to an aspect of the image forming apparatus of the present invention, the control means quantizes a density level of an original image at the time of image formation.
According to an aspect of the image forming method of the present invention, whether the print medium is an OHP film or a regular sheet of paper is determined in the identifying step.
According to an aspect of the image forming method of the present invention, the controlling of printing is to control scan-line density at the time of image formation.
According to an aspect of the image forming method of the present invention, an output pulse of a laser beam at the time of image formation is controlled in the control step.
According to an aspect of the image forming method of the present invention, in a case where it is determined that the print medium is an OHP film in the identifying step, image formation is performed in the control step at print density such that the OHP film does not act as a diffraction grating due to a formed image.
According to an aspect of the image forming method of the present invention, the print density where the OHP film does not act as a diffraction grating is 250 Lines/inch or more.
Other objects and advantages besides those discussed above shall be apparent to those skilled in the art from the description of a preferred embodiment of the invention which follows. In the description, reference is made to accompanying drawings, which form a part thereof, and which illustrate an example of the invention. Such example, however, is not exhaustive of the various embodiments of the invention, and therefore reference is made to the claims which follows the description for determining the scope of the invention.